With today's glass fiber technology, it is possible to transmit volumes of data or information in optical transmission systems with approximately 10 Gbit/s. The special advantages of optical transmission are a low transmission loss and insensitivity to electromagnetic interferences. The basis for optical communication is essentially formed by light generating transmitter oscillators for example, internally or externally modulated laser diodes, the fiber and the receiver, for example photo diodes, possibly with amplifiers. For the transmission of large volumes of data, especially the DWDM (Dense Wavelength Division Multiplex) has become established. DWDM makes it possible to transmit simultaneously many wavelengths of light via a mutual (or one) fiber glass. For this purpose, several signals are combined or bundled for the transmission of signals and information and transmitted essentially simultaneously via a line.
The maximum line length for data rates of 10 Gbit/s is in the range of 100 km and, among other things, limited by fiber attenuation. In addition to fiber attenuation, the dispersion of the fiber material becomes noticeable, especially if large transmission paths are involved. This results in a dispersion of the group velocity of the individual wavelengths of light. The extremely short light pulses of a high speed signal of approximately 10 Gbit/s correspond with a certain spectral width, which results in pulse broadening of the temporal signal, especially also during low dispersion.
The attenuation and dispersion effects, as well as linear and non-linear effects reduce the transmission of high signal rates. However, to allow for a transmission of large volumes of data (currently, optical data transmissions with rates of up to approximately 100 Gbit/s are being tested), it is required to know or compensate interferences (or disturbances) that affect transmission.
Currently, among other things, attempts have been made to compensate the dispersion by counteracting opposing dispersion with optical elements. Typically this involves appropriately designed and integrated fiber portions. Since this is some kind of “static” compensation, as it were, it is not possible to record, for example, time-dependent or “dynamic,” such as thermic and/or mechanical, changes of the transmission path. As a result, when using data rates of 40 Gbit/s, it is necessary to perform additional active dispersion compensation for fine tuning. In order to perform this fine tuning correctly, the actual dispersion or change of dispersion has to be known.